For ultrasound imaging apparatuses, the following methods are available: one method is to apply an external force to a subject via an ultrasound wave transmit/receive surface of an ultrasound probe, determine a displacement of each point in the subject by using a correlation operation of ultrasound wave received signal data on a pair of adjacent time-series frames, measure a distortion by performing spatial differentiation on the displacement, and form an image of distortion data, and another method is to form an image of elastic modulus data such as a Young's modulus of a tissue from a stress distribution caused by an external force and distortion data (e.g., Japanese Patent Application Laid-Open No. 5-317313 and Japanese Patent Application Laid-Open No. 2000-60853). The external force applied to the subject includes pressing and depressing the subject. Hereinafter, the external force will be simply referred to as “pressing” Ultrasound received signal frame data at a given time reflects, as information, the configuration and arrangement of tissues in the subject at that time. In a method for obtaining tissue elasticity information by using ultrasound waves, first by using ultrasound received signal data of a pair of frames obtained at regular time intervals, a displacement of each part of a tissue is calculated. The displacement is caused by a pressure applied between the regular time intervals. Then, displacement information undergoes spatial differentiation, so that distortions are calculated for all points in a region of interest (ROI) and an elastic image is constructed and displayed. With an elastic image obtained based on such distortion/elasticity modulus data (hereinafter, referred to as elastic frame data), the hardness of a tissue can be measured and displayed.
In order to obtain high-quality elastic image data, it is preferable to apply a pressure causing a tissue of interest to have a distortion of about 0.5% to 1%. In a time phase when a distortion within a proper range is not applied, extracted elastic image data is disturbed. When obtaining ultrasound received signal frame data of a pair of frames at regular time intervals, a high pressing speed at a certain time causes large distortion of a tissue at that time, and a low pressing speed at a certain time causes small distortion of a tissue at that time. Therefore, the quality of two or more elastic image data (particularly distortion image data) obtained in a series of pressing processes depends upon a pressing speed at a time when obtaining ultrasound received signal frame data of a pair of frames for constituting the elastic image data.
In elasticity imaging using a conventional ultrasound imaging apparatus, a tissue of interest is manually pressed by an ultrasound probe. Thus, it is difficult to keep pressing within a pressing speed range suitable for high image quality all the time in a series of pressing processes. Further, a pressing speed is not constant at respective times, so that a plurality of outputted elastic image data becomes temporally discontinuous and elastic images become discontinuous between frames. Further, it is not possible to avoid a movement of hands in the pressing process. A pressing direction varied between times also causes discontinuity of the elastic image data having been sequentially obtained. Therefore, the quality of an elastic image depends upon the technique of the operator.
At the time intervals for obtaining ultrasound received signal frame data, a tissue of interest is moved out of a measuring cross section by a pressure in the short axis direction of the probe, or displaced at high speed in the long axis direction or pressing direction of the probe, so that the tissue of interest may deviate from ROI set by the imaging apparatus. In this manner, due to an improper pressing direction or an excessive speed, ROI set by the imaging apparatus may have an error (correlation operation error) region, in which a correct displacement cannot be calculated. In a deep region where transmitted ultrasound waves can not reach due to attenuation and in a region with few ultrasound reflectors (a cyst and a lesion having a liquid therein), ROI set by the imaging apparatus may have an error (correlation operation error) region, in which a correct displacement cannot be calculated, because no received signal reflecting a property of the tissue of interest with sufficient intensity cannot be obtained. Moreover, ROI set by the imaging apparatus may include an error region where the calculation of a displacement is insignificant, due to the shape of the ultrasound probe and the pattern of a tissue of interest, for example, in a region where the ultrasound probe is not in contact with the subject. In these cases, a distortion image is not correctly displayed in the error region.
Furthermore, a region having a displacement close to 0 may be entirely distributed in ROI set by the imaging apparatus due to a pressing speed of 0 or an insufficient pressing speed. Such pressing speeds occur when a pressure is not applied to a tissue of interest at time intervals for obtaining a pair of ultrasound received signal frame data and a pressing speed on a tissue of interest is too low. In this case, a distortion image indicating distortions calculated using the displacement has a low contrast over the ROI.
In elastic imaging using the conventional ultrasound imaging apparatus, an image is constructed and displayed for all the measuring points of the set ROI without evaluating whether data (distortion or modulus of elasticity) outputted as an arithmetic result is worth displaying or not (reliability and quality of data). Therefore, even though image information on a region calculated under improper conditions is not worth displaying, the image information cannot be discriminated from information worth displaying. As a result, an elastic image of one frame is constructed such that a region worth displaying and a region not worth displaying are mixed, reducing the reliability of an elastic image.
In the conventional ultrasound imaging apparatus, whether a pressing operation for applying an external force from a body surface to a living tissue of the subject is proper or not is not considered. Hence, it is not always possible to obtain a proper elastic image.
In other words, an elastic image is obtained by determining a modulus of elasticity from a displacement (distortion) of each part of a living tissue and a pressure or the like, and imaging a distortion pattern qualitatively or a modulus of elasticity quantitatively based on frame data of two tomographic images different in time series. The tomographic images have been obtained by applying an external force to a living tissue. A distortion of each part of a living tissue varies according to a pressing operation including a pressure, a pressing speed, a pressing time, a pressing direction and the like. Without a certain distortion difference between adjacent two frames, a proper elastic image cannot be generated.
Particularly, for simplicity, an external force is applied by pressing the ultrasound probe to a body surface of the subject in many cases, though an external force may be applied by mechanical device. A pressing state is considerably changed by the feeling of the operator, and thus it is not always possible to obtain a proper elastic image. Similarly, because of variations among subjects, even when an operation is performed in a uniform pressing state, a proper elastic image cannot be always obtained.
Further, a pressing direction and the way to press may cause lateral displacement on a living tissue. Also in this pressing operation, an elastic image may include disturbance (noise) caused by lateral displacement and a proper elastic image may not be obtained.
The present invention is devised in view of these circumstances. An object of the present invention is to provide an ultrasound imaging apparatus which can stably form a high-quality elastic image in a given time phase during elastic imaging. An object of the present invention is also to provide an ultrasound imaging apparatus, by which when typical and ideal data is hard to obtain in elastic imaging, a region of image information including an elastic value not worth displaying is recognized as, e.g., noise, and an elastic image reflecting the information is constructed, high-quality elastic imaging is enabled. Moreover, an object of the present invention is to provide the operator with pressing operation information for obtaining a proper elastic image.